1. Field of the Invention
This invention relates generally to recording and reading data from magnetic storage media and, more particularly, to servo control systems that maintain the position of a magnetic head relative to tracks in magnetic storage media.
2. Description of the Related Art
The recording and reading of data in tracks on magnetic storage media requires precise positioning of magnetic read/write heads. The read/write heads must be quickly moved to, and maintained centered over, particular data tracks as recording and reading of data takes place. The magnetic heads can record and read data as relative movement occurs between the heads and the magnetic storage media in a transducing direction. The heads are moved from track to track across the width of the tracks in a translating direction, which is perpendicular to the transducing direction.
For example, a recordable disk typically contains concentric data tracks and is rotated beneath a magnetic head. The direction of rotation defines the transducing direction. Radial movement from track to track defines the translating direction. A magnetic tape typically contains data tracks that extend along the length of the tape, parallel to the tape edges, in the transducing direction. In magnetic tape helical scan systems, however, the tape is moved beneath heads that are moved at an angle across the width of the tape, the diagonal direction defining the transducing direction.
Storage devices that read and record data on magnetic media typically use servo control systems to properly position the data heads in the translating direction. The servo control systems derive a position signal from a servo magnetic head that reads servo control information recorded in servo tracks on the storage media. Typically, the servo control information comprises two parallel but dissimilar patterns. The servo head follows the boundary between the two dissimilar servo patterns, which are recorded in alignment with the data tracks. When the servo head is centered relative to the boundary between the servo patterns, the associated read/write head is centered relative to the data track.
The servo patterns might comprise bursts of half-width magnetic flux transitions, extending halfway across the servo track, that have different phases or frequencies. These patterns are often referred to as "half tracks" because a single servo position is defined by an adjacent pair of the patterns. Generally, the servo head has a width greater than or equal to approximately one-half servo track. With a half-width servo head it is readily possible to determine which direction to move the head for centering up until the head has moved more than one-half track off center. Servo heads that are less than one-half track width would not be able to determine which direction to move as soon as the head was completely over one half of the servo track or the other. Servo heads that are greater than one-half track width are most commonly used with imbedded servo systems, which use the same read head for servo and for data. With such systems, every other pattern is made different to avoid the problem of the head running into an adjacent track pattern, which then would not be able to determine which direction in which to move.
An alternative to the half-track servo control approach is described in U.S. Pat. No. 3,686,649 to Behr, which describes a disk drive servo control system that uses servo control information comprising lines of magnetic flux transition that extend across a servo track width at two different angles from a line parallel to a disk radius. A pair of such transition lines define a control zone in the form of a symmetric trapezoid. A control head detects a positive-going pulse generated by a first transition and a negative-going pulse generated by a second transition. The signal thus generated comprises a pulsed position signal that can be compared with a reference signal to indicate how far the control head has deviated from the servo track centerline. The system is said to permit more than 200 tracks per inch on a storage disk. Nevertheless, there is a demand for disk storage devices and tape storage devices of greater and greater storage density. For example, conventional disk drives can provide 5000 tracks per inch.
The half-track servo control approach has been found to be generally satisfactory for direct access storage devices, such as disk drives. Tape storage systems operate under unique characteristics that increase the difficulty of providing higher storage densities. In magnetic tape storage systems, the storage media/magnetic head interface is not as clean as the environment typically found in disk systems and, unlike most disk systems, the magnetic tape runs substantially in contact with the magnetic head. The relatively dirty environment and continuous contact between the media and the head, as well as the relatively large width of the servo head, produces significant wear and scratching of both the media and the servo head and produces localized build-up of contaminants on the surfaces of both. As a result, the spatial response of the servo head to the servo control information changes with time, both gradually as a result of wear over time and suddenly as a result of interaction with contaminant debris.
Changes in the servo head spatial response cause errors in the position signal, so that a position signal can indicate no track misregistration when the servo head actually is displaced from the servo track centerline. Errors in the position signal are typically difficult to detect from the position signal itself. As a result. redundant servo tracks are often used for increased reliability, wherein the servo control system uses the position signal data only if the data from two or more redundant tracks agree. Redundant servo tracks reduce the tape storage media surface area available for data recording and requires more heads and supporting electronics.
From the discussion above, it should be apparent that there is a need for a servo control system that is especially suited to the magnetic tape environment, that reduces the magnitude of position signal error due to wear on the servo head and debris, and that permits position signal errors to be detected more easily. The present invention satisfies this need.